The present invention relates to polymer films. Specifically, the present invention relates to multilayer metallized polyolefin film structures having improved barrier properties and reduced water vapor transmission and oxygen transmission properties.
Generally, in the preparation of a film from granular or pelleted polymer resin, the polymer is first extruded to provide a stream of polymer melt, and then the extruded polymer is subjected to the film-making process. Film-making typically involves a number of discrete procedural stages including melt film formation, quenching and windup. For a general description of these and other processes associated with film-making, see K R Osborn and W A Jenkins, Plastic Films. Technology and Packaging Applications, Technomic Publishing Co., Inc., Lancaster, Pa. (1992).
An optional part of the film-making process is a procedure known as xe2x80x9corientation.xe2x80x9d The xe2x80x9corientationxe2x80x9d of a polymer is a reference to its molecular organization, i.e., the orientation of molecules relative to each other. Similarly, the process of xe2x80x9corientationxe2x80x9d is the process by which directionality (orientation) is imposed upon the polymeric arrangements in the film. The process of orientation is employed to impart desirable properties to films, including making cast films tougher (higher tensile properties). Depending on whether the film is made by casting as a flat film or by blowing as a tubular film, the orientation process requires substantially different procedures. This is related to the different physical characteristics possessed by films made by the two conventional film-making processes: casting and blowing. Generally, blown films tend to have greater stiffness, toughness and barrier properties. By contrast, cast films usually have the advantages of greater film clarity and uniformity of thickness and flatness, generally permitting use of a wider range of polymers and producing a higher quality film.
Orientation is accomplished by heating a polymer to a temperature at or above its glass-transition temperature (Tg) but below its crystalline melting point (Tm), and then stretching the film quickly. On cooling, the molecular alignment imposed by the stretching competes favorably with crystallization and the drawn polymer molecules condense into a crystalline network with crystalline domains (crystallites) aligned in the direction of the drawing force. As a general rule, the degree of orientation is proportional to the amount of stretch and inversely related to the temperature at which the stretching is performed. For example, if a base material is stretched to twice its original length (2:1) at a higher temperature, the orientation in the resulting film will tend to be less than that in another film stretched 2:1 but at a lower temperature. Moreover, higher orientation also generally correlates with a higher modulus, i.e., measurably higher stiffness and strength.
Biaxial orientation is employed to more evenly distribute the strength qualities of the film in two directions. Biaxially oriented films tend to be stiffer and stronger, and also exhibit much better resistance to flexing or folding forces, leading to their greater utility in packaging applications.
It is technically quite difficult to biaxially orient films by simultaneously stretching the film in two directions. Apparatus for this purpose is known, but tends to be expensive to employ. As a result, most biaxial orientation processes involve apparatus which stretches the film sequentially, first in one direction and then in the other. Again for practical reasons, typical orienting apparatus stretches the film first in the direction of the film travel, i.e., in the longitudinal or xe2x80x9cmachine directionxe2x80x9d (MD), and then in the direction perpendicular to the machine direction, i.e., the lateral or xe2x80x9ctransverse directionxe2x80x9d (TD).
The degree to which a film can be oriented is dependent upon the polymer from which it is made. Polypropylene, polyethylene terephthalate (PET), and nylon are highly crystalline polymers that are readily heat stabilized to form dimensionally stable films. These films are well known to be capable of being biaxially stretched to many times the dimensions in which they are originally cast (e.g., 5xc3x97 by 8xc3x97 or more for polypropylene).
The film-making process can also include vacuum metallization to obtain a metal-like appearance and to enhance the barrier characteristics of a film. Further, the film-making process can include coating a film to impart superior characteristics to the film and methods of coating are well known in the art. Most known methods provide for coating a film after it has been biaxially oriented. However, most known methods do not provide for coating a metallized film.
Attempts have been made in the past to provide metallized films having enhanced barrier properties. For example, U.S. Pat. No. 5,525,421 issued to Knoerzer discloses a metallized film including an oriented polypropylene substrate layer having at least one surface of a coating of a vinyl alcohol homopolymer on which there is a metal layer. Also, U.S. Pat. No. 4,692,380 issued to Reid discloses a metallized film including a homopolypropylene core layer having on one of its surfaces a corona-treated propylene-ethylene copolymer layer and a metal coating applied on the corona-treated layer.
Accordingly, it is one of the purposes of this invention, among others, to provide multilayer metallized polyolefin films having improved barrier properties and reduced water vapor transmission and oxygen transmission properties, without requirement for chemical additives such as cross-linking agents, and without requirement for supplemental processing steps such as irradiation of the film or lamination.
The present invention is directed to film structures in which a biaxially oriented polyolefin substrate is metallized and then coated. The present invention provides for a multilayer film structure having improved barrier properties and reduced water vapor transmission and oxygen transmission properties. The multilayer film structure includes a polyolefin substrate having a first and second side, a metal layer adjacent to the first side of the substrate and substantially coextensive therewith and a coating layer adjacent to the metal layer and substantially coextensive therewith. The coating layer includes a coating selected from the group consisting of polyvinylidene chloride (PVdC), polyvinyl alcohol (PVOH) and acrylic coating.
Preferably, the coating is a highly crystalline PVdC. Further, the coating layer is preferably from about 3.0 wt % to about 22.0 wt % of the film structure.
The metal layer includes a metal selected from the group including aluminum, gold, silver, chromium, tin and copper. In addition, the metal layer is preferably less than about 0.1 wt % of the film structure.
The substrate is preferably from about 77.0 wt % to about 96.0 wt % of the film structure.
In a preferred embodiment, the multilayer film structure of the present invention further includes a primer layer between the metal layer and the coating layer. Preferably, the primer layer includes a primer selected from the group including polyethylene imine, urethane and epoxy. Further, the primer layer is preferably from about 0.01 wt % to about 1.0 wt % of the film structure.
In another preferred embodiment, the multilayer film structure of the present invention further includes a skin layer between the substrate and the metal layer. The skin layer includes a polyolefin selected from the group including polypropylene homopolymer, ethylene propylene random copolymer, ethyl vinyl alcohol copolymer, high density polyethylene (HDPE), medium density polyethylene (MDPE), linear low density polyethylene (LLDPE), low density polyethylene (LDPE), ethylene propylene butylene terpolymer and propylene butylene copolymer.
In another preferred embodiment, the multilayer film structure of the present invention further includes a skin layer adjacent to the second side of the substrate. The skin layer includes a polyolefin selected from the group including ethylene propylene copolymer, ethylene propylene butylene terpolymer, propylene butylene copolymer, ethylene propylene impact copolymer, ethylene methyl acrylate copolymer, ethylene vinyl acetate copolymer, HDPE, MDPE, LDPE and LLDPE. Optionally, the skin layer is coated with a coating selected from the group including PVDC, acrylic coating, ethylene acrylic acid copolymer and PVOH.
The films of the present invention can be widely used in food packaging applications due to their superior barrier properties as films utilized in food packaging must be as resistant as possible to the transmission of moisture, air and deleterious flavors.
The multilayer metallized polyolefin film structures of the present invention have improved flavor and aroma barrier properties and reduced water vapor transmission and oxygen transmission properties. These properties result from the combination of a metal layer and a coating layer in the film structures of the present invention. The barrier properties of the film structure of the present invention are also preserved since the coating layer protects the metal layer from scratching and mechanical degradation. These properties make these films an excellent alternative to laminated films currently used in food packaging.
These and other advantages of the present invention will be appreciated from the detailed description and example which are set forth herein. The detailed description and example enhance the understanding of the invention, but are not intended to limit the scope of the invention.